1. Field of the Invention
The present invention relates to a motor-driven or an electrical power steering system (hereinafter referred to as motor-driven power steering system) and, more particularly a sample-hold circuit that is controlled by an abnormality detecting function.
2. Description of the Related Art
A motor-driven power steering system for assisting operation of the steering wheel in which steering wheel operation is detected by means of steering torque to control motor rotation speed according to the steering torque is well known, as disclosed in JP-B2-2678377.
Such a motor-driven power steering system has a sample-hold circuit for detecting motor current, which has a microcomputer. The sample-hold circuit cyclically samples motor current values according to a timing signal provided by the microcomputer and holds the motor current value for a preset period.
When the current value is held by the sample-hold circuit, the current value, which is an analog signal, is converted to a digital signal by a separate A-D converter, thereby controlling operation of the motor.
Although an abnormality detecting means is equipped with, the sample-hold circuit does not correctly operate if the timing signal is not sent to the sample-hold circuit due to disconnection or breaking of a timing signal wire. For example, if a command current value is larger than a detected current value that is detected a current sensor and held by a capacitor before the disconnection or breaking, a microcomputer increases the command current value further to narrow the difference between the command current value and the detected current value because the detected current value gradually decreases as the capacitor discharges. Accordingly the motor current is excessively increased, so that power assisting is not correctly carried out.